This invention concerns the use of specially crosslinked polyacrylamide, water absorbing polymers for reducing lost circulation when aqueous drilling fluids are used and a method for reducing such lost circulation. These polymers are able to reach various levels of areas of fissures and thief zones before absorbing enough water and to expand to plug those fissures and thief zones.
Drilling fluids or drilling muds as they are sometimes called, are slurries of solids used in the drilling of wells in the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and the pumping of the drilling fluid.
For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the drilling fluid may be lost into the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through pores in the rock matrix surrounding the borehole.
Most wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation. Unfortunately, formation conditions frequently are encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake established.
A variety of different substances are often pumped down well bores in attempts to reduce the large losses of drilling fluid to fractures and the like in the surrounding formation. Different forms of cellulose are preferred materials by some persons. Other substances which are pumped into well bores to control lost circulation are: almond hulls, black walnut hulls, bagasse, dried tumbleweed, kenaf, paper, coarse and fine rice. These and a number of other prior art materials are described in U.S. Pat. No. 4,498,995.
Another process that is employed to close off large lost circulation areas is referred to in the art as gunk-squeeze. In that process, a quantity of a powdered bentonite is mixed into diesel oil and pumped down the well bore. Water injection follows and the water and bentonite will harden to form a gunky, semi-solid mass which can reduce lost circulation. This process must be carefully controlled; first the bentonite must be kept dry until it reaches the desired location in the well and then problems frequently occur in trying to mix the bentonite and water in the well. This method is disclosed in U.S. Pat. No. 3,082,823.
Many of the methods devised or proposed to limit lost circulation involve the use of water expandable materials. The black walnut hulls and other nut hulls have been found to undergo some swelling when used in this operation. (see Glowka et al, Journal of Petroleum Technology, March 1990, pages 328 and ff.) Cremeans in U.S. Pat. No. 4,217,965 teaches the use of compressed, swellable, pelletized cottonseed hulls in combination with cottonseed meal, bentonite, residual lint and a surface active agent. The advantage of this compressed material is that it does not expand quickly when added to the drilling fluid but only after it is well down the well bore.
Another swellable material sometimes used is a water expandable clay such as bentonite which may be mixed with another ingredient to form a viscous paste or cement. U.S. Pat. No. 2,890,169 discloses a lost circulation fluid made by forming a slurry of bentonite and cement in oil. The slurry is mixed with a surfactant and water to form a composition comprising a water-in-oil emulsion having the bentonite and cement dispersed in the continuous oil phase. As this composition is pumped down the well bore, the oil expands and flocculates the bentonite which, under the right conditions, forms a cake on the wellbore surface in the lost circulation area. Hopefully, the settling of the cake of the walls will cause the emulsion to break thus causing the previously emulsified water to react with the cement to form a solid coating on the cake. But such a complex process can easily go wrong.
U.S. Pat. No. 3,448,800 discloses another method wherein a water soluble polymer is slurried in a non-aqueous medium and injected into a well. Another slurry of a mineral material such a barite, cement or plaster of paris is subsequently injected into the well to mix with the first slurry to form a cement-like plug in the wellbore.
U.S. Pat. No. 4,261,422 describes the use of an expandable clay such as bentonite or montmorillonite which is dispersed in a liquid hydrocarbon into the well. After injection the clay will expand on contact with water in the formation. Thus it is hoped that the expanding clay will close off water producing intervals, but not harm oil producing intervals.
A similar method is disclosed in U.S. Pat. No. 3,078,920 which uses a solution of a polymerized methacrylate dissolved in a non-aqueous solvent such as acetic acid, acetic anhydride, propionic acid and liquid aliphatic ketones such as acetone or methyl-ethyl ketone. The methacrylate will expand on contact with formation water in the water producing formations in the well.
It has also been proposed to mix bentonite with water in the presence of a water soluble polymer which will flocculate and congeal the clay to form a much stronger and stiffer cement-like plug than will form when bentonite is mixed with water alone. U.S. Pat. No. 3,909,421 discloses such a fluid made by blending a dry powdered polyacrylamide with bentonite followed by mixing the powder blend with water. U.S. Pat. No. 4,128,528 claims a powdered bentonite/polyacrylamide thickening composition prepared by mixing a water-in-oil emulsion with bentonite to form a powdered composition which rapidly becomes a viscous, stiff material when mixed with water. U.S. Pat. Nos. 4,503,170; 4,457,594; 4,445,576; 4,442,241; and 4,391,925 teach the use of a water expandable clay dispersed in the oily phase of a water-in-oil emulsion containing a surfactant to stabilize the emulsion and a polymer dispersed in the aqueous phase. When the emulsion is sheared, it breaks and a bentonite paste is formed which hardens into a cement-like plug. The patent discloses the use of such polymers as polyacrylamide, polyethylene oxide and copolymers of acrylamide and acrylic or methylacrylic acid.
U.S. Pat. No. 4,124,748 discloses a cross-linked copolymer of a vinyl ester and an ethylenically unsaturated carboxylic acid or derivative thereof that can absorb about 200 to 800% of its weight in water and expand substantially in volume when doing so. U.S. Pat. No. 3,997,484 is typical of a group of related patents by Fanta, Doane and co-workers at the USDA Northern Regional Laboratory which describe various water swellable polymers made by grafting acrylonitrile onto starch and then hydrolyzing the resultant copolymers. These polymers are capable of absorbing large amounts of water very rapidly with the concomitant swelling have been used in a specific form as lost circulation materials as described below. The rights to use these patents have been granted to several companies and some are commercially available. Advances on this technology have been regularly made to the present time. For example, U.S. Pat. No. 5,065,822 teaches the use of a polyvalent metal such as iron which is formed as the ingredients are passing down the borehole to contact a previously placed starch hybrid polymer, thus crosslinking the polymer to make the polymer formation impermeable.
Another highly water absorbent copolymer which expands upon water absorption is described in U.S. Pat. No. 4,320,040. The described compound is derived by polymerizing acrylic acid and/or methacrylic acid in the presence of polyvinyl alcohol followed by neutralization and a heat treatment.
Highly absorbent spongy polymers which may absorb large quantities of water and hydrocarbons causing an increase in volume are disclosed in U.S. Pat. No. 3,878,175. These are copolymers of an acyl acrylate and a heterocyclic N-vinyl monomer containing a carbonyl functionality and a crosslinking agent in the presence of a hydrophobic liquid diluent. U.S. Pat. No. 4,182,677 discloses that natural and synthetic rubbers will also swell in size upon absorbing water.
Many of the approaches mentioned here and later have used crosslinked polyacrylamides as the water absorbing polymer. One of the first disclosures of such materials in U.S. Pat. 3,247,171 which teaches the preparation and then hydrolysis of a crosslinked polacrylamide polymer. Variations in method of polymerization as well as the type and depth of hydrolysis were shown to vary the properties of the products such as rate of absorption or water and propensity to retain the water absorbed. Many variations of that theme have been subsequently reported with U.S. Pat. No. 4,559,074 being quite notable. This mildly ionic crosslinked polyacrylamide has a high water absorbtivity, retains the water well and can be manufactured reproducibly.
Much research has been dedicated to the use of encapsulants and methods to produce encapsulated products of various types. Although this is typical of the pharmaceutical and cosmetic field, the use of encapsulated materials to provide useful, swellable lost circulation materials have not been overlooked. This technique is sometimes useful in preventing the swelling of water absorbing polymers and other materials prior to their reaching the thief zone or otherwise interfering with the drilling operation itself.
An invention using encapsulation of materials for use in boreholes is described in U.S. Pat. No. 4,078,612. The patent describes an explodable material encapsulated in natural gums slurried in a liquid vehicle. The material is pumped into the formation around the wellbore and exploded to decrease permeability.
The use of bentonite encapsulated within a water insoluble polymeric coating within a water-insoluble polymeric coating has been disclosed for lost circulation control. U.S. Pat. No. 2,836,555 describes bentonite encapsulated within a polymeric coating having a tiny hole drilled therethrough. When the encapsulated bentonite is pumped down the wellbore, water will seep through the hole in the encapsulant causing the bentonite to swell and ultimately rupture the coating.
Another U.S. Pat. No. 4,036,301 describes an encapsulated material useful in cementing a well, wherein a cement accelerator is encapsulated in a waxy material and placed within a highly retarded cement slurry. The cement slurry is pumped into the well with the encapsulated accelerator. After proper placement of the cement, circulation is decreased so that the temperature of the cement fluid approaches the bottom hole temperature of the well and melts the encapsulated material, freeing the accelerator which sets the cement.
A similar technique is disclosed in U.S. Pat. No. 4,362,566 which suggests encapsulating one component of a two or more component adhesive of cement mixture so that hardening will not start until the encapsulated component is freed from its reaction-preventing casing.
Similarly, U.S. Pat. No. 4,664,8116 teaches introducing a water absorbent polymer encapsulated by a selected waxy substance which prevents the polymer from expanding by absorbing water until it reaches the lost circulation zone. This encapsulation material then dissolves or melts at the desired temperature within the borehole, thereby releasing the water absorbent polymer to absorb water and expand to seal the lost circulation zone.
Each of the above discussed methods has sufficient disadvantages making it necessary to continue the search for an effective, efficient and readily controlled method of reducing or eliminating lost circulation without undue interruption of the drilling process. The methods mentioned generally go to great lengths to prevent the swollen lost circulation material from being finally generated in the drilling zone thereby impeding the flow of the drilling mud or making it so viscous that the drilling becomes inefficient. When some of the materials mentioned above do reach the lost circulation zone they must be somehow activated to absorb water and swell still in the proper region which against difficult to control. In addition some of the materials are difficult or remove when desired and other because of their nature are removed too soon.
In accordance with the method of this present invention, the selected polymers are in granular form which can be readily introduced into the wellbore along with the aqueous drilling fluid and the granules will pass through the wellbore substantially in a non-swollen state. A polymer of suitable particle size can be selected which can proceed down the wellbore to the appropriate depth where a fissure may have occurred. Once in the area of lost circulation the granules can accumulate, absorb water and swell to form a xe2x80x9csoft gelxe2x80x9d, watertight plug thereby sealing the porous formation. In addition the materials can be used in conjunction with other lost circulation materials such as ground walnut shells or other aforementioned swellable or fibrous materials to reinforce the plug or improve the economics of the sealing operation. Finally, the products of this invention are only slowly biodegraded in contrast with starch or cellulose based materials but can be readily removed when desired by the addition of appropriate acids or metallic salts.
In accordance with the method of this present invention, the selected polymers are in granular form which can be readily introduced into the wellbore along with the aqueous drilling fluid and the granules will pass through the wellbore substantially in a non-swollen state. Once in the area of lost circulation the granules can accumulate, absorb water and swell to form a xe2x80x9csoft gelxe2x80x9d, watertight plug thereby sealing the porous formation. To accommodate the depth of the thief zone the user can select from among combinations of particle size and chemical structure of the recited polymers. In addition each material can be used in conjunction with other lost circulation materials such as ground walnut shells, compacted cottonseed hulls or other swellable or fibrous materials to reinforce the plug or improve the economics of the sealing operation. Finally, the products are only slowly biodegraded in contrast with starch or cellulose based materials as well as many other readily available polymers. They can be readily removed, however when desired, by the addition of appropriate acid or metallic salt solutions.
Several objects and advantages of this invention are to present simple products and methods for their use as lost circulation materials which may be used in conjunction with on-shore, off-shore, water drilling, coring and geothermal wells for oil and gas. The lost circulation additives discovered are specific, suitably crosslinked polyacrylamides of somewhat similar chemical but important chemical composition and of various particle size distributions which can be added directly to the drilling fluid and thereby be transported to the appropriate depth of the lost circulation area where they will then absorb large quantities of water and swell drastically in the borehole. This swelling substantially closes the fissures and pores through which drilling fluid would be lost. In addition the polymers can be used in conjunction with other swellable or non-swellable lost circulation materials which may also be added to the borehole. The xe2x80x9csoft gelxe2x80x9d formed by the polyacrylamides can then act alone or in conjunction with the other additives to close any rather large fissures or pores.
Unlike the materials described in Background Section, there is no need to encapsulate, compound, slurry or otherwise pre-treat these lost circulation materials before use. Also there is no need to interrupt or otherwise change the drilling procedure, wait an undue period or depend on the adjustment of the temperature in the borehole for the lost circulation materials to take effect,
Still further objects and advantages will become apparent from a consideration of the ensuing description.
Crosslinked polyacrylamides became available in the mid-1970""s and the polymers made by graft polymerization of acrylonitrile with starch followed by hydrolysis became available about the same time. Many variations in the properties of each could be made by varying the conditions of synthesis. Both classes of polymers found use in various applications where absorption of water is important. A large market developed and still exists for their use in disposable diapers and other similar incontinence products. Another major market which evolved was for their use as soil amendments to retain moisture for grasses, trees, crops and the like which increases the survival rate as well as appearance, crop yield, root structure, etc. Since both classes of copolymers are hydrolyzed from their copolymer precursors, they can be obtained as salts of various anions such as but not limited to sodium, potassium or magnesium and each have unique physical properties.
The physical appearance of the two classes of copolymers as initially prepared is significantly different by nature of the chemical compounds used to synthesize them. The starch graft copolymers are usually fine powders as they leave the manufacturing process but some can be manufactured in flake or film form. The crosslinked polyacrylamide copolymers as manufactured generally appear as semi-crystalline or amorphous clumps of rather large size. These materials then can be ground by various means to the particle size most suited to the application intended. For example the materials described in U.S. Pat. No. 4,559,074 mentioned above can be obtained in various particle sizes and particle size distributions. For example one can obtain the crosslinked polyacrylamide described in this patent from Broadleaf Industries Inc. of Costa Mesa, Calif. in several particle size ranges. The products named Broadleaf P4(copyright) can be obtained in the range of 0.50-2.50 millimeters as their standard size, 0.075-0.500 millimeters as their fine size and less than 0.075 millimeters as the ultra-fine size (often called Root Dip).
More recently Broadleaf Industries Inc. has made available a copolymer of acrylamide and sodium acrylate similarly crosslinked as in the ""074 Patent and in the particle size range of xcx9c1.0-5.0 millimeters which is called Broadleaf P4+ and which hydrates at a slower rate than regular P4 thus allowing it to flow further down a borehole before swelling. It could be available in other particle size ranges to suit a given drilling situation. Of course there may be some limit on the size of the given particles depending upon the dimensions of a particular wellbore operation.
It must be realized that similar polymers from other suppliers could be similarly used.
Polymers for use in disposable diapers are required to be able to absorb urine which is primarily water and as rapidly as possible to immobilize the urine as quickly as possible. In addition the ability of such materials to biodegrade in a reasonably short time is also an advantage. The starch graft copolymers are ideal for this application because of their small particle size which hastens the water absorption and the very nature of the starch base itself; starch based materials in general are easily biodegraded. Variations in the rate of absorption of water by commercially available polyacrylamides can be seen in the Examples which follow. The difference in biodegradability is shown by M. S. Johnson, Arab. Gulf J. Scient. Res. 3 (2) 745-750 (1985). Therefore if the starch graft copolymers do get to the borehole properly and do swell there, they can biodegrade rather rapidly and once again open the fissures and pores allowing circulation material to be lost again. Although it is sometimes desirable to remove the caked lost circulation material, the rate and time when this occurs via biodegradation depends on the microbiolgical environment of the borehole which cannot be readily controlled.
Unfortunately the very properties of the starch based copolymers and some crosslinked polyacrylamides which make them ideal for use in disposable diapers make them difficult to use as lost circulation materials. A review of Superabsorbent Polymers, Science and Technology, Bucholz and Peppas, American Chemical Society, 1994 pps. 88-111, clearly shows that the use of such polymers in disposable diapers has driven the technology and the similar development of such materials for other uses such as in agriculture and the drilling industry has been minor. Fortunately Clarke in the U.S. ""074 Patent deals with this trend by inventing a crosslinked polyacrylamide specifically designed to have the properties more suitable for the agricultural industry. The careful examination of these materials and their partially hydrolyzed derivatives led to the trial of these products for use as lost circulation materials as described in this invention. When working with the three grades of Broadleaf P4 mentioned above, it was noticed that the finely ground known as Ultra-fine material absorbed water rapidly to form a very viscous slurry which would make it difficult to pump that material to the borehole and for it to congregate there to form a cake. The material designated as Fine forms clumps when first contacting water and requires undue agitation to break apart the clumps so that all of the material will become swollen.
More recently examination of the similarly crosslinked acrylamide/sodium acrylate copolymer led to their trial as Lost Circulation Materials.
Therefore since the very finely divided particles of either type of polymer will absorb water so rapidly from the drilling fluid used to carry them into the borehole, they swell on the way to the borehole. This increases the viscosity of the drilling fluid considerably, or they form agglomerated clumps which also interferes with the movement of the drilling fluid and traps solids from that fluid which, in turn, modifies its characteristics. In addition since the swelling occurrs before settling in the borehole and coating the fissures and voids therein, the swollen copolymer particle can be carried back out of the borehole to the surface and accumulate there. It is for this reason that earlier so much effort was placed on finding encapsulants for the starch based copolymers or otherwise keep them from swelling and the same is true for some other water absorbing polymers, Typical examples of this were described above in U.S. Pat. Nos. 4,664,516 and 4,836,940.
It was therefore determined that the standard Broadleaf P4(copyright) or that type of product in larger particle size such as Broadleaf P4+ were most useful for this present invention. The Broadleaf P4+ product can be use advantageously when a fissure at a further distance down a borehole needs to be sealed and this advantage is based on the slightly different chemical composition of the polymer from that of Broadleaf P4 and also the fact that the large particle size presents a larger surface area to the available water thus slowing the absorption of water but not diminishing the total water absorption. Of course other materials having similar properties might be used if they should become available.
Crosslinked Polyacrylamide polymers and copolymers in general are less biodegradable than the starch based materials as mentioned above; those designed for use in disposable diapers and the like are the most biodegradable. Crosslinked Polyacrylamides also vary in the amount of water they will absorb which in turn reflects the amount of swelling they will undergo, and in the amount of water they can retain over time. This is shown by Wang and Gregg in J. Amer. Soc. Hort Sci. 115(6):943-948 (1990) and demonstrated below. Lot to lot variation has been found also in the various other crosslinked Polyacrylamide copolymers although some might still be suitable for use as lost circulation materials if properly tested before use. The products mentioned above and sold as Broadleaf P4(copyright) and Broadleaf P4+ were especially designed to avoid the problems of other crosslinked polyacrylamides and do have a reasonable rate of water absorption, also a high level of water retention and possess resistance to rapid biodegradation. This resistance to biodegradation was illustrated by Johnson, Arab. Gulf J. scient. Res. 3(2) 745-750 (1985). However a swollen cake of those materials can be removed when desired by adding a quantity of dilute hydrochloric acid, acetic acid, certain metallic salts and the like to the well bore whereupon the cake will completely dissolve or disintegrate as shown in the Examples below.
In the practice of the invention, the selected, suitable crosslinked Polyacrylamide copolymer or other suitable water absorbing polymer which will not interfere with the drilling fluid operation or inadvertently biodegrade, is added to the drilling fluid via a hopper or pouring the material by hand from the bag or container or by any other means suitable to adding granular particulate matter. The material is then carried down the borehole where it absorbs water, swells and the swollen material forms a xe2x80x9csoft gelxe2x80x9d on the fissures. The selection and amount of copolymer to be added to the drilling fluid can readily be adapted to the characteristics of the particular borehole.
Other suitable materials may be mixed with the polymer in an amount equal to or up to 150 times the weight of said polymer and the mixture similarly put into the drilling fluid and allowed to proceed to the borehole. Similarly the materials can be added sequentially to the drilling fluid and allowed to proceed to the borehole. This is illustrated in the Examples.
Such suitable materials may or may not be swellable and may include but are not limited to walnut hulls, cottonseed hulls, thermoset rubber, coal, graphite, other mixed nut shells, perlite, kenaf and the like.
To remove a formed cake of these lost circulation materials in the borehole, a quantity of dilute hydrochloric acid or other suitable acid is added to the drilling fluid in the proper amount necessary to develop the necessary acid strength to dissolve or otherwise cause the destruction of the polymer cake in the borehole. Fifteen percent (15%) hydrochloric acid is convenient to use but the concentration need not be limited to this. Alternatively, solutions of various metallic salts such as sodium chloride, potassium chloride, calcium chloride and the like may be added to the borehole which after contacting the cake will shrink the cake so that it will fragment and be pumped from the borehole.
The following examples are intended only to illustrate the various aspects of the invention which is defined by the claims.